1. Field of the Invention
The present invention relates to an inverter, and particularly, to a discharging control apparatus of a switching device for an inverter.
2. Background of the Invention
In general, an inverter is widely used to drive alternating current (AC) loads, such as a three-phase alternating current (AC) motor or the like. The inverter serves to convert a commercial AC voltage into a direct current (DC) voltage and to invert the DC voltage into AC voltage again. Wherein the converted DC voltage is switched by a switching device to be then inverted to the AC voltage again, so as to be supplied to a load, such as an AC motor. And the inverter can accurately control the velocity of the three-phase AC motor.
The switching device, namely, a power switching device, is usually configured by employing an insulating gate bipolar transistor (abbreviated IGBT hereinafter). In the inverter, particularly, in a large capacity inverter, the capacity of current which is switched by the switching device to be outputted to a load is very large. Thus, even during the normal operation of the inverter, a high level of spike voltage may be generated when the switching device is turned off. Such spike voltage may damage the switching device.
Hence, it is required to reduce the high level of spike voltage which is generated when the switching device switches a DC voltage in the large capacity inverter.
Hereinafter, an inverter to which the present invention is applied and a driving circuit of a switching device for an inverter will be described with reference to FIG. 1.
FIG. 1 is a circuit view showing a configuration of a general three-phase inverter. Here, a reference numeral 100 denotes a switching circuit section for switching a DC voltage to output an AC voltage to an AC load 110, such as an AC motor. The switching circuit section 100 includes totally 6 switching devices IGBT1 to IGBT6, one pair for each phase, in case where the load 110 is a three-phase load. Each two of 6 switching devices IGBT1 to IGBT6 are paired so as to be parallel-connected between a power source terminal VDC and a ground. Connection points of the switching device pairs (i.e., (IGBT1, IGBT2), (IGBT3, IGBT4) and (IGBT5, IGBT6)) are connected to the load 110.
A reference number 120 denotes a pulse width modulation (PWM) signal generating circuit for switching the switching device pairs (i.e., (IGBT1, IGBT2), (IGBT3, IGBT4) and (IGBT5, IGBT6)). For example, under the assumption that the switching circuit section 100 includes 6 switching devices IGBT1 to IGBT6, the PWM signal generating circuit 120 generates 6 PWM signals.
A reference numeral 130 denotes a plurality of gate driving units. The plurality of gate driving units 130 respectively switch the switching devices IGBT1 to IGBT6 of the switching circuit section 100 according to the PWM signals generated from the PWM signal generating circuit 120.
In the three-phase inverter with such configuration, the PWM signal generating circuit 120 generates PWM signals having high level voltage (logical 1) and low level voltage (logical 0) in an alternating manner. Each pair, namely, each two of the generated PWM signals, have a phase difference of 180° therebetween, and the paired PWM signals have a phase difference of 120° therebetween.
The generated PWM signals are input in the plurality of driving units 130. The driving units 130 apply or stop applying driving voltages to gates of the respective switching devices IGBT1 to IGBT6 of the switching circuit section 100 according to the PWM signals.
Each of the switching devices IGBT1 to IGBT6 is selectively turned on or off in a repetitive manner according to whether the gate driving voltage is applied. That is, the pairs of switching devices (i.e., (IGBT1, IGBT2), (IGBT3, IGBT4) and (IGBT5, IGBT6)) are alternately turned on or off in the repetitive manner. The pairs of switching devices (i.e., (IGBT1, IGBT2), (IGBT3, IGBT4) and (IGBT5, IGBT6)) are turned on or off with a phase difference of 120° therebetween.
In cooperation with the turn-on or turn-off of the plurality of switching devices IGBT1 to IGBT6, a DC voltage of the power source terminal VDC is converted (inverted) into an AC voltage, and the inverted AC voltage is supplied to the load 110 so as to drive the load 110.
Hereinafter, a configuration of the driving unit of the switching device according to the related art, including a discharge circuit of the switching device for the inverter, will be described with reference to FIG. 2.
As shown in FIG. 2, each gate driving unit 130 of the switching device according to the related art includes one charging path and a discharging path. The charging and discharging paths are connected in parallel between the PWM signal generating circuit 120 and the gate of each switching device IGBT as shown in FIG. 1. The charging path includes an NPN type transistor TR1 and a turn-on resistor R1 for restricting a charging current. The discharging path includes a PNP type transistor TR2 and a turn-off resistor R2 for restricting a discharging current.
The operation of the gate driving unit 130 of each switching device according to the related art having such configuration will now be described.
If the PWM signal generated from the PWM signal generating circuit (see 120 in FIG. 1) is a high level signal, the NPN type transistor TR1 is turned on such that a DC voltage of a (+) power source terminal Vcc is supplied to a base of the switching device IGBT via the turn-on resistor R1, thereby turning the switching device IGBT on. If the PWM signal generated from the PWM signal generating circuit (see 120 in FIG. 1) is a low level signal, the NPN type transistor is turned off and simultaneously the PNP type transistor TR2 is turned on, such that the DC voltage Vcc is not supplied to the base of the switching device IGBT any more, thereby turning the switching device IGBT off. In cooperation with the turn-off of the switching device IGBT, a gate voltage VGE having charged between an emitter of the switching device IGBT and the gate thereof is discharged to a (−) power source terminal Vee via the turn-off resistor R2 and the PNP type transistor TR2.
A discharging operation when a switching device is turned off, in a gate driving unit of a switching device according to the related art, will now be described with reference to FIG. 3.
First, as shown in FIG. 3(a), a low level PWM signal generated from the PWM signal generating circuit 120 is applied to the gate driving unit 130 at time t1 so as to turn the switching device IGBT off. Accordingly, the PNP type transistor TR2 is turned on, whereby a discharging current IG of the switching device IGBT is drastically increased to be maintained at a certain level. The discharging current IG starts to be decreased at time t2, a time at which the switching device IGBT is substantially turned off so as to become 0 (zero). Here, indicating the discharging current IG by a minus (−) value in FIG. 3(a) is intended to represent that the discharging current is discharged via the (−) power source terminal Vee upon the discharging.
FIG. 3(b) is a waveform view showing the change in a voltage VGE between gate and emitter (hereinafter, called ‘gate voltage) of a switching device IGBT upon turning the switching device IGBT off. As shown in FIG. 3(b), the gate voltage VGE of the switching device IGBT is drastically decreased at the time t1 when the low level PWM signal generated from the PWM signal generating circuit 120 for turning the switching device IGBT off is applied to the PNP type transistor TR2. After being maintained at a certain level, such gate voltage VGE is then drastically decreased at the time t2 when the switching device IGBT is substantially turned off to become 0 (zero).
FIG. 3(c) is a waveform view showing the change in a voltage VCE between emitter and collector (hereinafter, emitter-collector voltage) of a switching device IGBT upon turning the switching device IGBT off.
As shown in FIG. 3(c), the emitter-collector voltage VCE of the switching device IGBT is gently increased to a voltage close to 0 (zero), during a period from the time t1 when the low level PWM signal generated from the PWM signal generating circuit 120 for turning the switching device IGBT off is applied to the PNP type transistor TR2 to the time t2 when the switching device IGBT is substantially turned off. The emitter-collector voltage VCE of the switching device IGBT is drastically increased at the time t2 when the switching device IGBT is substantially turned off, so as to become a DC voltage of the (+) power source terminal Vcc.
In the large capacity inverter having the gate driving units according to the related art, a current capacity switched by the switching device IGBT is very high, even when the switching device IGBT is turned off under the state that a normal current flows in the switching device IGBT, a high level spike voltage may be generated.
In order to reduce such spike voltage, a method of increasing the resistance value of the resistor 2 for discharging a gate current of the switching device IGBT is employed. However, the increase in the resistance value of the resistor R2 causes a turn-off time of the switching device IGBT to be extended, resulting in an increase in the loss of the switching device IGBT
In addition, in the gate driving unit of the switching device for the inverter according to the related art, when turning the switching device off under the state that an over-current flows in the switching device, the spike voltage is much increased, thereby damaging the switching device (e.g., burning it out).